Lasers arranged for light intensive activities, such as photolithography, can be expensive to operate and maintain. Part of this cost relates to stringent requirements for conditioning laser beams and another part of this cost relates to the limited service lives of such lasers between required refurbishing. The beam-conveying optics, which are often located in an isolated environment within a laser housing or other enclosed pathway, participate in such activities as beam amplification, beam shaping, wavelength selection, and bandwidth control. High-power lasers, such as Excimer lasers and frequency-doubled solid-state lasers, generate high concentrations of light energy that can progressively damage the beam-conveying optics.
While it is generally possible to replace one or more of the beam-conveying optics following a predetermined degradation in laser performance, such as a reduction in laser output power associated with reduced transmissivity of the optics, downtime associated with such reconditioning can be expensive, both as a refurbishing exercise and as a loss of production. For example, a four to five percent reduction in transmissivity of an individual optical component is often enough to require its replacement.
The service lives of high-power lasers are generally measured in terms of billions of shots, referring to a number of pulses generated by the laser. For example, Excimer lasers used in photolithographic operations have service lifetimes of around five to ten billion shots before some refurbishing becomes necessary. That is, such lasers require significant servicing on a regular basis depending upon their amount of use.
Ongoing application trends demand ever higher average power, which can further reduce the expected service lives of the lasers and add significantly to their cost of ownership. Higher photon intensities associated with shorter wavelengths, now commonly within the deep ultraviolet spectrum, limit the choices of optical materials meeting transmissivity requirements. Birefringence and other optical performance measures also disqualify many optical materials at the shorter wavelengths.
Laser performance is also temperature sensitive because most of the available optical materials tend to expand with the application of heat, i.e., have significant coefficients of thermal expansion. Thus, the optical components are specially designed to operate at a constant elevated operating temperature of the laser, e.g., around 40 degrees Celsius. Attempts have been made to increase the service lives of the optical components by choosing among the available materials and by reducing impurities within the materials, which are expected to increase absorption. However, the material choices even among different manufacturers remain limited and further reductions in impurities have only marginally improved results.